1. Field of the Invention
The present invention generally relates to a component operating with acoustic waves, and in particular to an electro-acoustic component with a piezoelectric substrate on which an electrode structure that includes aluminum is applied.
2. Description of the Related Art
Components that operate with acoustic waves include surface acoustic wave components (SAW components) and FBAR resonators (film bulk acoustic resonator). SAW filters and filters made from FBAR resonators are increasingly being used in the front end of mobile radio devices as transmitting and receiving filters. The entire transmission capacity of the mobile radio device flows through the transmission filter, such that the electrode structures of these components are exposed to a maximum current load. Moreover, due to the increasing miniaturization of mobile telecommunications devices and due to the introduction of new mobile radio standards, the power density in the components is ever higher. In addition to requiring a high current load capacity of the electrode structures, the acoustic waves in these devices subject the electrodes to a strong mechanical stress that can in the long term lead to destruction of the electrode structure and, as a result, to the failure of the component or, respectively, the filter.
Tests of mechanically and electrically severely stressed filters show that the electrodes are destroyed by material migration of the electrode material (acousto- and electromigration). These material migrations manifest themselves as the formation of voids and, on the surface of the electrode structures, as in the formation of outgrowths that are known as hillocks. FIG. 1 shows an electrode structure of a SAW component damaged by acousto-migration. The strip-shaped electrode structures ES (here shown in cross-section) are applied on a substrate S. From the originally rectangular cross-section of the electrode strips, the outgrowths H have formed via acousto-migration, the electrode structures being capable of occurring both on the side of the electrode structures and on the upper surface of the electrode structures. The voids V form parallel to this. An electrode structure which has thus been modified possesses a changed geometry whose electromechanical properties are changed. If outgrowths H are formed between two oppositely charged electrode fingers, these can be the origin of a short or an arc-over between the two electrode fingers. In addition to the modified properties of the damaged electrode structure, an electrical arc-over between different polarized electrode fingers typically leads to the destruction and thus to the total failure of the filter. In contrast, the modification (increasing under stress load) of the electromechanical properties of electrode structures leads to a continuous change of the resonance frequency and to an unwanted rise in the insertion damping.
Aluminum is typically used as an electrode material in SAW components. To limit the damaging acousto-migration, alloys of, for example, AlCu, AlMg, AlCuMg, AlCuSc, AlZrCu, AlTi, AlSi and others are increasingly being used. These alloys reduce the migration, in that they form deposits at the aluminum crystal borders that block the diffusion paths. Dependent on the metal or element added to the aluminum, this leads to the added element being fixed in the aluminum crystals, reinforcing the crystals and reducing the material migration in the electrodes. More detailed information concerning such electrode structures or, respectively, of the materials used for them are located in the printed publications U.S. Pat. No. 5,774,962; R. Takayama et al., “High Power Durable Electrodes for GHz Band SAW Duplexers”; and R. Takayama et al., “High Power SAW Filters with New Al—Sc—Cu/Ti/Al—Sc—Cu/Ti Electrodes.” By suppressing the acousto-migration, both the continuous change of the resonance frequency and the insertion damping and the danger of the short formation and the arc-overs are counteracted. However, the alloys used have a higher specific electrical resistance than non-alloyed metals. This impairs the filter performance and effects a higher insertion damping. Moreover, the spontaneous heating is increased, which in turn has a negative effect on the output tolerance of the electrode structures and thus the filter. The maximum output tolerance is therewith also limited with the proposed alloys.
Furthermore, it has been proposed to use layer systems made from different metals and aluminum alloys instead of a homogenous layer made from aluminum or an aluminum alloy. For this, one or more intermediate layers made from copper, magnesium, titanium, chromium or other metals are used between aluminum and/or aluminum alloy layers. These intermediate layers block the diffusion of the aluminum through these layers and reduce the effect of the continuous change of the resonance frequency and the insertion damping. This is, for example, proposed in the printed publications Y. Satoh et al., “Ladder Type SAW Filter and Its Application to High Power SAW Devices”; and U.S. Pat. No. 5,909,156.
In order to increase the bonding of the electrode structures on the substrate, a bonding layer made from titanium can be applied between the undermost aluminum or aluminum alloy layer, as is known, for example, in U.S. Pat. No. 5,844,347; and European Patent Publication EP 0803919 B1. This titanium bonding layer improves the <111>—texture and therewith the output compatibility.
Furthermore, it has already been proposed to use pure copper to produce electrode structures, or alternatively to use a layer system of copper-aluminum or copper-aluminum-copper. Independently of this, it has been proposed to generate large-area passivation layers over the electrode structures in order to protect the electrode structures from external effects such as corrosion and to prevent the migration. Silicon dioxide, silicon nitride and aluminum nitride have already been proposed as materials for such a passivation layer, for example in the printed publication J. Kim et al., “Passivation Layer Effects on Power Durability of SAW Duplexer.”.